Defibrillation pacing circuitry

ABSTRACT

Electrical circuit componentry is switchable into a defibrillator circuit to deliver a constant pacing current to a patient. The circuitry may include a constant current source inserted in a leg of the defibrillator circuit or a resistor of selected value inserted between a high voltage source and the high side of a defibrillator circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The invention of the present application may find application insystems such as are disclosed in U.S. patent application entitled“SUBCUTANEOUS ONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONALPACER,” having Ser. No. 09/663,607, filed Sep. 18, 2000, pending, andU.S. patent application entitled “UNITARY SUBCUTANEOUS ONLY IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser. No.09/663,606, filed Sep. 18, 2000, pending, of which both applications areassigned to the assignee of the present application, and the disclosuresof both applications are hereby incorporated by reference.

[0002] In addition, the foregoing applications are related to the U.S.patent application entitled “DUCKBILL-SHAPED IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND METHOD OF USE,” U.S. patent applicationentitled “CERAMICS AND/OR OTHER MATERIAL INSULATED SHELL FOR ACTIVE ANDNON-ACTIVE S-ICD CAN,” U.S. patent application entitled “SUBCUTANEOUSELECTRODE FOR TRANSTHORACIC CONDUCTION WITH IMPROVED INSTALLATIONCHARACTERISTICS,” U.S. patent application entitled “SUBCUTANEOUSELECTRODE WITH IMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION,”U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FORTRANSTHORACIC CONDUCTION WITH HIGHLY MANEUVERABLE INSERTION TOOL,” U.S.patent application entitled “SUBCUTANEOUS ELECTRODE FOR TRANSTHORACICCONDUCTION WITH LOW-PROFILE INSTALLATION APPENDAGE AND METHOD OF DOINGSAME,” U.S. patent application entitled “SUBCUTANEOUS ELECTRODE FORTRANSTHORACIC CONDUCTION WITH INSERTION TOOL,” U.S. patent applicationentitled “METHOD OF INSERTION AND IMPLANTATION FOR IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTERS,” U.S. patent application entitled“CANISTER DESIGNS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS,” U.S.patent application entitled “RADIAN CURVED IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTER,” U.S. patent application entitled“CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA ANDORIENTATION THEREOF,” U.S. patent application entitled “BIPHASICWAVEFORM FOR ANTI-BRADYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLECARDIOVERTER-DEFIBRILLATOR,” and U.S. patent application entitled“BIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUSIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR,” the disclosures of whichapplications are hereby incorporated by reference.

FIELD OF THE INVENTION

[0003] The present invention relates to apparatus and methods useful inconnection with performing electrical cardioversion/defibrillation andoptional pacing of the heart.

BACKGROUND OF THE INVENTION

[0004] Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias in the atriaand/or ventricles. Typically, electrodes are employed to stimulate theheart with electrical impulses or shocks, of a magnitude substantiallygreater than pulses used in cardiac pacing.

[0005] Defibrillation/cardioversion systems include body implantableelectrodes that are connected to a hermetically sealed container housingthe electronics, battery supply and capacitors. The entire system isreferred to as implantable cardioverter/defibrillators (ICDs). Theelectrodes used in ICDs can be in the form of patches applied directlyto epicardial tissue, or, more commonly, are on the distal regions ofsmall cylindrical insulated catheters that typically enter thesubclavian venous system, pass through the superior vena cava and, intoone or more endocardial areas of the heart. Such electrode systems arecalled intravascular or transvenous electrodes. U.S. Pat. Nos.4,603,705, 4,693,253, 4,944,300, 5,105,810, the disclosures of which areall incorporated herein by reference, disclose intravascular ortransvenous electrodes, employed either alone, in combination with otherintravascular or transvenous electrodes, or in combination with anepicardial patch or subcutaneous electrodes. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and5,618,287, the disclosures of which are incorporated herein byreference. A sensing epicardial electrode configuration is disclosed inU.S. Pat. No. 5,476,503, the disclosure of which is incorporated hereinby reference.

[0006] In addition to epicardial and transvenous electrodes,subcutaneous electrode systems have also been developed. For example,U.S. Pat. Nos. 5,342,407 and 5,603,732, the disclosures of which areincorporated herein by reference, teach the use of a pulsemonitor/generator surgically implanted into the abdomen and subcutaneouselectrodes implanted in the thorax. This system is far more complicatedto use than current ICD systems using transvenous lead systems togetherwith an active can electrode and therefore it has no practical use. Ithas in fact never been used because of the surgical difficulty ofapplying such a device (3 incisions), the impractical abdominal locationof the generator and the electrically poor sensing and defibrillationaspects of such a system.

[0007] Recent efforts to improve the efficiency of ICDs have ledmanufacturers to produce ICDs which are small enough to be implanted inthe pectoral region. In addition, advances in circuit design haveenabled the housing of the ICD to form a subcutaneous electrode. Someexamples of ICDs in which the housing of the ICD serves as an optionaladditional electrode are described in U.S. Pat. Nos. 5,133,353,5,261,400, 5,620,477, and 5,658,321 the disclosures of which areincorporated herein by reference.

[0008] ICDs are now an established therapy for the management of lifethreatening cardiac rhythm disorders, primarily ventricular fibrillation(V-Fib). ICDs are very effective at treating V-Fib, but are therapiesthat still require significant surgery.

[0009] As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, especially children at risk ofcardiac arrest, the requirement of ICD therapy to use intravenouscatheters and transvenous leads is an impediment to very long termmanagement as most individuals will begin to develop complicationsrelated to lead system malfunction sometime in the 5-10 year time frame,often earlier. In addition, chronic transvenous lead systems, theirreimplantation and removals, can damage major cardiovascular venoussystems and the tricuspid valve, as well as result in life threateningperforations of the great vessels and heart. Consequently, use oftransvenous lead systems, despite their many advantages, are not withouttheir chronic patient management limitations in those with lifeexpectancies of >5 years. The problem of lead complications is evengreater in children where body growth can substantially altertransvenous lead function and lead to additional cardiovascular problemsand revisions. Moreover, transvenous ICD systems also increase cost andrequire specialized interventional rooms and equipment as well asspecial skill for insertion. These systems are typically implanted bycardiac electrophysiologists who have had a great deal of extratraining.

[0010] In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of a related therapy, theautomatic external defibrillator (AED). AEDs employ the use of cutaneouspatch electrodes, rather than implantable lead systems, to effectdefibrillation under the direction of a bystander user who treats thepatient suffering from V-Fib with a portable device containing thenecessary electronics and power supply that allows defibrillation. AEDscan be nearly as effective as an ICD for defibrillation if applied tothe victim of ventricular fibrillation promptly, i.e., within 2 to 3minutes of the onset of the ventricular fibrillation.

[0011] AED therapy has great appeal as a tool for diminishing the riskof death in public venues such as in air flight. However, an AED must beused by another individual, not the person suffering from the potentialfatal rhythm. It is more of a public health tool than a patient-specifictool like an ICD. Because >75% of cardiac arrests occur in the home, andover half occur in the bedroom, patients at risk of cardiac arrest areoften alone or asleep and can not be helped in time with an AED.Moreover, its success depends to a reasonable degree on an acceptablelevel of skill and calm by the bystander user.

[0012] What is needed therefore, especially for children and forprophylactic long term use for those at risk of cardiac arrest, is acombination of the two forms of therapy which would provide prompt andnear-certain defibrillation, like an ICD, but without the long-termadverse sequelae of a transvenous lead system while simultaneously usingmost of the simpler and lower cost technology of an AED. What is alsoneeded is a cardioverter/defibrillator that is of simple design and canbe comfortably implanted in a patient for many years.

[0013] Moreover, it has appeared advantageous to the inventor to providethe capability in such improved circuitry to provide a signal suitablefor pacing when the circuitry is not operating in a defibrillation mode.

SUMMARY OF THE INVENTION

[0014] Accordingly, the invention relates in various aspects to methodsand apparatus for selectively converting a defibrillator circuit orcircuit for delivering a defibrillating pulse to a patient intocircuitry suitable for providing a constant current, useful, e.g., inpacing applications.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a better understanding of the invention, reference is nowmade to the drawings where like numerals represent similar objectsthroughout the figures and wherein:

[0016]FIG. 1 is a schematic view of a conventional defibrillatorcircuit;

[0017]FIG. 2 is a circuit schematic of an illustrative embodiment of thepresent invention;

[0018]FIG. 3 is a circuit schematic of an alternate embodiment;

[0019]FIG. 4 is a circuit schematic of a second alternate embodiment;

[0020]FIG. 5 is a circuit schematic illustrating one approach tocontrolling switching in the embodiment of FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0021]FIG. 1 illustrates a conventional “H-bridge” defibrillator circuit11. The circuit 11 includes a capacitor C₁ which is charged to a highvoltage and four switches H₁, H₂; L₁, L₂. The capacitor C1 and switchesH₁, H₂; L₁, L₂ are used to create either a monophasic voltage pulse or abiphasic voltage pulse (FIG. 2) across a patient represented byresistance R_(PAT). In various applications, the switches H₁, H₂; L₁,L₂, may be MOSFETs, IGBTs, or SCRs (silicon controlled rectifiers).

[0022] To create a biphasic waveform such as that shown in FIG. 2, afirst pair of switches, e.g., H₁ and L₂, may be closed to create a firstpositive pulse 13. Then all of the switches, H₁, H₂; L₁, L₂, are turnedoff during a “center pulse” delay period d₁. At the end of the delayperiod d₁, the switches H₂ and L₁ are both closed, thereby reversing thecurrent through the patient R_(PAT) to produce the negative voltagepulse 17. Typically, digital logic is employed to control the sequencingof the switches H₁, H₂; L₁, L₂. In such case, the order of the pulsescan be inverted, i.e., the negative going pulse 17 can be producedbefore the positive going pulse 13. In illustrative applications, thedirection of the pulses 13, 17 is, e.g., 1 to 20 millisecond and theinter-pulse delay d₁ is, e.g., one millisecond.

[0023]FIG. 3 illustrates circuitry which may operate as a defibrillatorcircuit during a first selected interval and as a constant currentsource during a second selected interval. The constant current may beuseful, for example, in providing a “pacing” current to a patientR_(PAT).

[0024] As in FIG. 1, the high side switches H₁, H₂ employed in FIG. 3may be IGBTS, MOSFETS, SCRS, or other suitable switches. Such high sideswitches H₁, H₂ may be controlled in any suitable manner such as, forexample, with pulse transformers, opto-couplers, photo-voltaicgenerators, or in accordance with the teachings of the application filedon even date herewith on behalf of the same inventor and entitledSimplified Defibrillator Output Circuit, herein incorporated byreference. Digital logic suitable for controlling such circuitry toachieve switching may comprise a programmed microprocessor,microcontroller, discrete logic, or other forms of digital logiccontrol.

[0025] In the circuit of FIG. 3, a resistor R₁ is inserted in serieswith the emitter or the source leg of a first low side transistor Q₂,which is preferably an IGBT or MOSFET. Similarly, a resistor R₂ isinserted in series with the emitter or source leg of the second low sidetransistor Q₁. A constant voltage is applied across the resistor R₁ viaa voltage source S₁, which applies a voltage V_(DRIVE) to the gate (orbase) of the first low side transistor Q₂. During operation of thecircuit of FIG. 2 as a defibrillator, the transistors Q₁, Q₂ serve thepurposes of low side switches, e.g., L₁, L₂ of FIG. 1, and the resistorsR₁, R₂ are switched out of the circuit by suitable means, e.g., switchesSW₁ and SW₂. During pacing operation of the circuit of FIG. 3, asuitable switching signal S₁ is applied to switch resistor R₁ into thecircuit.

[0026] In an illustrative application of the circuitry of FIG. 3, thelow side transistors Q₁, Q₂ may be high voltage IGBT or MOSFETs, rangingfrom 500 volts to 3,000 volts capacity or greater. In the circuit ofFIG. 2, the voltage across the resistor R₁ is defined by the equation:

V _(R1) =V _(DRIVE) =V _(T)  (1)

[0027] where V_(T) is the fixed (constant) threshold voltage of the lowside transistor Q₁. Thus, if V_(DRIVE) is 15 volts, and V_(T) is in therange of 2-6 volts, V_(R1) is in the range of 13 to 9 volts.Accordingly, a constant voltage is applied across the resistor R₁,resulting in a constant current I_(RI) through the resistor R₁, andhence through the patient R_(pat).

[0028] As those skilled in the art may appreciate, the threshold voltageV_(T) of the transistor Q₁ may vary from device to device. Hence, it istypically necessary to calibrate the circuit in production. Incalibrating a circuit like that of FIG. 2, a known voltage is appliedand the current through R₁ is measured, typically resulting in a largeoffset, which is compensated for by the system software.

[0029] In order to avoid calibration, the voltage source S₁ may beconstructed using a feedback circuit employing an operational amplifierA₁ as shown in FIG. 4. The op-amp A₁ is connected to directly drive thelow side transistor Q₂, which may comprise, e.g., a MOSFET or IGBT. Useof the operational amplifier A₁ removes the uncertainty of the thresholdvoltage V_(T) SO that the current that passes through the resistor R₁ isequal to simply V_(DRIVE) divided by R₁. Thus, one can either drive thetransistor Q₂ with a voltage source and calibrate the system for theV_(T) of the transistor Q₂ or use an op-amp circuit to remove the errorcreated by the threshold voltage V_(T) of the transistor Q₂.

[0030] During constant current source operation of the circuit of FIG.3, the appropriate high side switch is on to permit current flow. Inaddition, the capacitor voltage V_(C) needs to be appropriately selectedaccording to a number of considerations. First, the current that isprogrammed to go through the patient will generate a voltage V_(PAT)across the patient. Then, in order to make the current source work, thevoltage compliance V_(COMP) of the current source must be appropriatelyset. In the case of FIG. 3, the voltage compliance V_(COMP) is thevoltage V_(R1) across the resistor R₁ plus the minimum operating voltageV_(T) of the low side transistor Q_(2.) Accordingly, the minimum voltageV_(HV) across the capacitor C₁ to current source is defined by therelation:

V _(HV)(min.)=V _(PAT) +V _(COMP)  (2)

[0031] The higher V_(HV) is above V_(HV) (min.), the closer the currentsource will approach an ideal current source. Another consideration insetting V_(HV) is power consumption.

[0032] The amount of current I_(R1) can be varied by varying the voltageV_(DRIVE) or by switching in different resistors, e.g., in series withor for R₁. From an implementation point of view, it is less attractiveto switch in a resistor because such switching requires addingtransistors or other switching devices. It is more efficient to simplyvary the voltage V_(DRIVE). Suitable logic circuitry may be provided toselect the value of V_(DRIVE). A DAC (digital to analog converter) isone example of such logic circuitry. As those skilled in the art willappreciate, a DAC is a circuit that generates different voltages inresponse to corresponding digital codes provided to it. Such a DAC couldbe used to drive either an input of the op-amp A₁ (as illustrated inFIG. 4) or the input (gate) of the transistor Q₁. As noted above, anadvantage of the op-amp A₁ is that it removes the V_(T) term from theV_(HV) equation. Particular parameter ranges for circuitry as configuredin FIGS. 2 and 3 include 1 to 50 ohms for the resistance R₁ and 1 to 20volts for a V_(DRIVE) resulting in a current ranging from 0 to 500milliamps.

[0033] Another illustrative circuit for implementing a current source isillustrated in FIG. 5. This circuit employs a resistor R₃ connectedbetween the high voltage capacitor C₁ and the high side switches H₁, H₃.The resistor R₃ is switched out of the circuit by a switch SW₃ fordefibrillator operation and into the circuit for pacing.

[0034] The circuit of FIG. 5 is somewhat more energy wasteful but willwork with the use of a high voltage switch for SW₃. In the circuit ofFIG. 5, the switches H₁, H₂; L₁, L₂ are manipulated so as to place theresistor R₃ in series with the output. The amount of current may then beselected by the voltage to which the capacitor C₁ is charged. As anexample, assuming the patient resistance R_(PAT) varies from 30-150ohms, selecting a resistor R₃ of anywhere from 500-5000 ohms, i.e., aresistance that is much larger than that of the patient, results in anapproximation of an ideal current source. The approximation is:$\begin{matrix}{i = \frac{{VH}_{V}}{R_{3} + R_{PAT}}} & (3)\end{matrix}$

[0035] While creation of a current source according to FIG. 5 isrelatively easy, switching the circuit to the defibrillation mode ismore complex. As shown in FIG. 6, a high voltage switch SW₃ is connectedacross the series resistor R₃ to switch R₃ out of the circuit in orderto enter the defibrillation mode. Since the high voltage switch SW₃ is afloating switch, a high side driver 19 is also needed. Theseconsiderations render the circuit of FIG. 6 more difficult to implementin an implantable device.

[0036] In contrast, the circuits of FIGS. 3 and 4 require a switch,e.g., SW₁ to switch to the defibrillation mode, but the switch SW₁ doesnot have to be a high voltage switch. Instead, the switch SWi need onlybe a smaller, low voltage device having the capacity to pass thedefibrillation current. In an illustrative circuit, there may be on theorder of only 10 volts across SW₁, which is advantageous.

[0037] Thus, only a low voltage switch need be used in the circuits ofFIGS. 3 and 4. No low voltage driver is necessary since the switch SW₁is referenced to ground and can therefore be driven directly. A highside driver circuit is unnecessary. In either of the circuits of FIG. 3or FIG. 4, the voltage V_(DRIVE) is preferably implemented by a DAC,either connected to directly drive the resistor R₁ (FIG. 3) or to drivethe resistor R₃ through an op-amp A₁ (FIG. 4).

[0038] Provision of a constant current has the advantage of maintaininga constant current density across the heart, irrespective of theelectrode interface impedance.

[0039] While the present invention has been described above in terms ofspecific embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. On the contrary, the followingclaims are intended to cover various modifications and equivalentmethods and structures included within the spirit and scope of theinvention.

What is claimed is:
 1. The apparatus comprising an H-bridgedefibrillator circuit; and means switchable into and out of said circuitfor causing a constant current to flow through a patient connected tosaid circuit.
 2. The apparatus of claim 1 wherein said means for causingcomprises means for imposing a constant voltage across a resistancelocated in the low side of said circuit.
 3. The apparatus of claim 2wherein said means for imposing comprises a constant voltage source. 4.The apparatus of claim 2 wherein said voltage source applies a constantvoltage to a control terminal of a switching device.
 5. The apparatus ofclaim 4 wherein said resistance is located in a leg of said switchingdevice.
 6. The apparatus of claim 3 wherein said constant voltage sourcecomprises a digital to analog converter (DAC).
 7. The apparatus of claim6 wherein said constant voltage source further comprises a feedbackcircuit.
 8. The apparatus of claim 4 wherein said constant voltagesource comprises a digital to analog converter (DAC).
 9. The apparatusof claim 8 wherein said constant voltage source further comprises afeedback circuit.
 10. The apparatus of claim 1 wherein said means forcausing comprises a resistance connected between a high voltage sourceand a high side of said defibrillator circuit.
 11. The apparatus ofclaim 10 wherein the value of said resistance is selected to be muchlarger than the resistance of said patient.
 12. The apparatus of claim10 further including a switch operable to switch said resistance in andout of said circuit.
 13. The apparatus of claim 12 further including ahigh side driver circuit for activating said switch.
 14. The apparatuscomprising: circuit means for delivering a defibrillating current pulseto a patient; and means switchable into and out of connection with saidcircuit means for causing a constant current to flow through saidpatient during a period when a current pulse is not being delivered. 15.The apparatus of claim 14 wherein said means for causing comprises meansfor imposing a constant voltage across a resistance located in a lowside of said circuit means.
 16. The apparatus of claim 15 wherein saidmeans for imposing comprises a constant voltage source.
 17. Theapparatus of claim 16 wherein said voltage source applies a constantvoltage to a control terminal of a switching device.
 18. The apparatusof claim 17 wherein said resistance is located in a leg of saidswitching device.
 19. The apparatus of claim 16 wherein said constantvoltage source comprises a digital to analog converter (DAC).
 20. Theapparatus of claim 19 wherein said constant voltage source furthercomprises a feedback circuit.
 21. The apparatus of claim 17 wherein saidconstant voltage source comprises a digital to analog converter (DAC).22. The apparatus of claim 21 wherein said constant voltage sourcefurther comprises a feedback circuit.
 23. The apparatus of claim 14wherein said means for causing comprises a resistance connected betweena high voltage source and a high side of said defibrillator circuit. 24.The apparatus of claim 23 wherein the value of said resistance isselected to be much larger than the resistance of said patient.
 25. Theapparatus of claim 23 further including a switch operable to switch saidresistance in and out of said circuit.
 26. The apparatus of claim 25further including a high side driver circuit for activating said switch.27. A method comprising the steps of: employing a defibrillator circuitto deliver a defibrillating energy pulse to a patient; and reconfiguringsaid circuit in response to at least one switching signal to deliver aconstant current to said patient.
 28. The method of claim 27 whereinsaid step of reconfiguring comprises switching a resistance into aselected portion of said defibrillator circuit.
 29. The method of claim28 wherein said resistance is switched into a leg of a low sideswitching device.
 30. The method of claim 29 wherein said resistance isswitched into series between a high voltage source and a high side ofsaid defibrillator circuit.
 31. The method of claim 28 further includingthe step of applying a constant voltage across said resistance.
 32. Themethod of claim 31 wherein a digital to analog converter is employed insaid step of applying a constant voltage.
 33. The method of claim 31wherein a feedback circuit is employed in said step of applying aconstant voltage.
 34. Electronic circuitry comprising: a switchingdevice connected in a circuit to assist in sending a current pulsethrough a patient; a resistance connected to be switchable in and out ofa leg of said switching device; and a source of constant voltage adaptedto be applied across said resistance.
 35. The circuit of claim 34wherein said source of constant voltage comprises a digital to analogconverter.
 36. The circuit of claim 34 wherein said source of constantvoltage comprises a feedback circuit.
 37. The circuit of claim 35wherein said source of constant voltage further comprises a feedbackcircuit.
 38. Electronic circuitry comprising: a high voltage source; aswitching device connected to assist in sending a current pulse througha patient; and a resistance switchable into and out of connectionbetween said high voltage source and said switching device and having avalue selected to cause a constant current to flow therethrough whensaid resistance is switched into said connection.
 39. The circuitry ofclaim 38 wherein said high voltage source comprises a capacitor.
 40. Thecircuitry of claim 39 further including a high side driver forcontrolling the switching of said resistance into and out of saidconnection.